What invalidates a contract of Mukatabah (manumission by installment)?

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A contract of Mukatabah is invalid if it stipulates an unknown consideration, a consideration due immediately, or a forbidden consideration, such as wine or swine. However, if an invalid condition is stipulated in the Mukatabah, the ruling is that the contract itself is not invalidated, but the condition is nullified, and the Mukatabah remains valid. This is the preferred opinion. Yet, a ruling attributed to Abu Abdullah suggests that Mukatabah conditioned upon a forbidden consideration is void, and manumission is not achieved by payment made under such a contract. This view is chosen by Abu Bakr, based on a report from Ahmad, who stated that if the Mukatabah is invalid but the required payment is made, the slave is manumitted, unless the contract was forbidden.